The bullet whizzed past Jiang Xiaobai's face by a hair's breadth, fortunately because he jumped sideways instead of straight up and down.
The glass sliding door behind the balcony was hit by the bullet and shattered instantly, scattering shards of glass everywhere.
The assassin on the opposite building clearly hadn't anticipated Jiang Xiaobai dodging his shot. Sniping takes time, and now that Jiang Xiaobai had spotted him, he had lost the prime opportunity for a clean shot. Abandoning the target and quickly leaving was his best option.
The professional killer Du Yuchun had hired was highly disciplined. When he missed the first shot, he immediately retreated. A killer's primary objective isn't necessarily to kill with one shot, but to protect themselves above all. There are many ways to kill, and if the first attempt fails, other opportunities can be created. There's no need for an all-out confrontation.